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Automatic programming system of EDM milling based on graphic exchange file

traditional EDM is mainly divided into two categories: EDM forming machining (SEDM) and WEDM. The key and one of the difficulties of forming processing is the manufacture of forming electrode. The design and manufacture of tools account for almost half of the total processing time, and the cost is also high. For many years, EDM researchers have been looking for ways to replace formed electrodes. EDM mill uses simple shape electrode to make forming movement according to a certain track, and processes through the discharge between tool electrode and workpiece. In this way, the production of shaped electrode is avoided and the productivity is improved. Accordingly, it also puts forward higher requirements for EDM machine tools, which requires the development of a special numerical control system. AutoCAD is the most widely used CAD software at present. It not only has rich two-dimensional drawing, commands and strong three-dimensional modeling functions. It also provides programming methods such as wired files, menu files, command files, etc. Its flexibility and openness determine that many applications choose it as the supporting platform for graphic design and its pre and post-processing. AutoCAD and its graphic format have become a de facto international industrial standard. In addition, AutoCAD can exchange data with other CAD systems or cam systems through standard data format, which is called drawingexchangefile (DXF file for short). To realize the integration of EDM milling cad/cam, it is necessary to extract useful part information from this file and convert these information into the NC program of EDM milling machine tool. 1 the DXF file of interface program design is ASC Ⅱ code text file. A typical DXF file is composed of six sections: title section, class section, table section, block section, entity section and object section. DXF files contain a great amount of information, but what is useful for NC programming is the entity section. Therefore, we only care about the content of the entity section. According to the data format of the entity section, the corresponding interface program is compiled to extract the geometric information of the drawing. However, the starting point coordinate obtained from the DXF file is the starting point of the first input drawing. The setting of the starting point of EDM milling needs to consider the stress state of the workpiece and the influence on the machining accuracy and surface roughness of the workpiece. It is often inconsistent with the starting point of the drawing. To solve this problem, we add the character o next to the starting point of processing when drawing the figure. To facilitate modification and reordering of entities in DXF files. We use a double linked list structure to store entity coordinate values

the flow chart of interface program is shown in Figure 1. 2 automatic discrimination of machining direction after obtaining the geometric information of the part, the tool path can be generated by compensating the tool radius (cylindrical tool). During tool radius compensation, the offset direction of the tool center is determined by the internal and external characteristics of the contour and the machining direction (the rotation direction of the machining closed loop). The internal and external characteristics of the contour are set during machining, and the machining direction must be determined according to the contour map and the tool feeding direction. Therefore, the discrimination of machining direction is the basis of solving tool compensation. The part contour is generally composed of straight lines and arcs, so we discuss the calculation of tool path of plane contour composed of these two graphics. Treat each side of the polygon as a vector, the vector direction is the cutting direction, and each vector is connected in turn according to the cutting order. As shown in Figure 1, m1m2, M7M8, m8m1. The arc contour takes the line between the starting point and the end point. The arrow in the figure indicates the tool travel direction. For the figure shown in Figure 2 (a), all points are convex points, and the discrimination method is relatively simple. By judging the cross product of two adjacent vectors at any vertex, the rotation direction of the machining closed loop can be determined. It is specified that the counterclockwise rotation of the processing closed loop is positive, and the clockwise rotation is negative. Then when the cross product of two vectors is positive, the processing direction goes counterclockwise; otherwise, it goes clockwise. However, for the contour shown in Fig. 2 (b), this method will not work. We designed the following two discrimination methods. The vertex rotation direction accumulation judgment method defines the vertex rotation direction as the cross product direction of two vectors adjacent to the vertex. For any plane figure, the processing direction is related to the rotation direction of each vertex, and N and K are material constants representing deformation strengthening. If the number of vertices with positive rotation direction is more than the number of vertices with negative rotation direction, the machining direction is positive; otherwise, it is negative. It can be verified that this method is effective for any plane graphics. However, when the number of vertices is large, the amount of computation is large. The pole discrimination method defines the pole of the polygon as the vertex with the smallest X coordinate. The closed-loop direction of machining can be obtained by judging the rotation direction of poles. Specifically, first, select poles and two adjacent vectors in the double linked list (if there are multiple poles, choose any one), and calculate their cross product. If the cross product is positive, the processing direction is positive (counterclockwise); otherwise, the processing direction is negative (clockwise). The pole discrimination method has nothing to do with the number of multilateral vertices, so the amount of calculation is small. 3 Calculation of tool path according to the machining direction judged above, the tool path can be calculated. Let the tool radius be DR and the unilateral discharge gap be D, then the offset of the tool center b=d+r

straight line contour arc contour for straight line contour, it is necessary to translate the straight line along the normal direction of the cutting direction by a distance B. given the points a (XA, ya) and B (XB, Yb) at both ends of the straight line, the straight line equation y=kx+c

after translation, the straight line equation is y=kx+c', C' can be calculated by the following formula

in which the positive and negative signs are selected according to the machining direction and the internal and external characteristics of the contour. Let the angle between the vector and the X axis be a, and table 1 shows the values in different cases. The compensation of arc contour is simpler than that of straight line, which only involves the increase or decrease of arc radius R. First, judge the direction of the arc. From the DXF file, we can obtain the coordinates of the starting and ending points of the arc, the radius of the arc, and the starting and ending angles. In this way, we can obtain the tangent vector at the starting and ending points of the arc. The cross product of the starting and ending tangent vectors determines the direction of the arc. The rule of judgment is: cross product is sure to maintain our impact machine well Positive, inverse circle: cross product is negative, and it is clockwise. After determining the arc direction, use table 2 to compensate the tool trajectory. After processing, it completes the automatic compensation of collision tool trajectory through information and wisdom. However, when two entities intersect to form sharp corners, it is also necessary to transition the sharp corners to avoid the interference of tools at the sharp corners or the discontinuity of tool tracks. We use the method of arc transition to pretreat the sharp corners, which solves this problem. According to the final tool path, the NC code can be generated after post-processing. Conclusion the DXF file output from AutoCAD provides a basis for cad/cam integration of EDM milling. According to the DXF file structure, this paper develops the interface software to extract entity information, and puts forward two methods to distinguish the machining direction. They can effectively judge the choice of halogen-free flame-retardant materials to reduce the odor of the air in the car and the rotation direction of the plane figure. According to the internal and external characteristics of the machining direction and contour, the tool path can be calculated. This lays the foundation for the generation of the final NC code. Using the method described in this paper, the tool trajectory can be obtained quickly and reliably. However, the tool path obtained according to the method in this paper is a theoretical path, and the tool loss in the machining process is not considered. In order to realize the integration of cad/cam in EDM milling and achieve better process effect, we also need to solve the problem of dynamic compensation of tool loss in the machining process

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